In a storage system, multiple storage servers are connected to each other to form a distributed system. A data storage task is performed in the system in coordination. However, a storage task may fail if a storage server is out of service. Therefore, to tolerate the out-of-service storage server, the storage system generally uses a manner of data redundancy storage. A typical approach is using a redundant array of independent disks (RAID). According to the RAID technology, a data block is divided into multiple data units, check units are obtained through exclusive OR calculations among the multiple data units, and the data units and the check units are stored in different storage servers. When a storage server is out of service, contents of data stored in the out-of-service storage server can be obtained through calculation by using the data units and the check units that are stored in storage servers that are not out of service, and therefore, that the out-of-service storage server is tolerable. During the out-of-service period of the storage server, because the contents of the data stored in the out-of-service storage server can be reacquired through calculation, the storage system is still allowed to provide a service, and the service in this case is referred to as a degraded service.
Generally, being permanently out of service means that a storage server in a storage system is faulty and is permanently excluded in the storage system. Being transiently out of service means that a storage server in a storage system is determined to be out of service within a period of time due to performance jitter or the like, but after the period of time, the storage server may rejoin the storage system.
If a storage server is out of service transiently, and another storage server is out of service shortly after the transiently out-of-service storage server rejoins the storage system, there might be permanent data loss.